1. Field of the Invention
The present invention relates to an injection control method and injection control device of a die-casting machine, which can be specifically used for injection control of a die-casting machine for manufacturing high-quality die-casting products without burr occurrence.
2. Description of Related Art
Conventionally, it is known that quality of die-casting molding products is largely affected by an injection speed and injection pressure in filling molten material into mold die. Especially, sufficient pressurization is required before the molten material solidifies and therefore a die-casting machine having double-stage driving cylinder devices for injection and boost has been used.
Generally, in the die-casting machine of this type, an injection plunger advances at a low speed and the molten material is started to be filled into the mold cavity while avoiding the molten material from getting choppy. When an end of the molten material reaches a gate portion of the die and pressure of filling injection cylinder device increases, the injection plunger is advanced at a high speed for avoiding the temperature of the molten material from being lowered to fill the mold cavity with the molten material rapidly.
In succession to the injection process, when the molten material is filled into the mold die to further raise the pressure of the injection cylinder device or when the injection plunger reaches a predetermined position corresponding to completion of filling, high pressure is applied to a boost cylinder device to conduct boosting process for increasing pressurizing force of the injection pressure in the mold die.
Specific arrangement of the conventional double-stage cylinder type die-casting machine will be described below.
In FIG. 6, molten material 92 to be filled in the mold cavity 91 is supplied to injection sleeve 93 of a die-casting machine 90. An injection plunger 94 is driven by a filling injection cylinder device 95 to inject the molten material 92. After completing filling process, hydraulic oil at a backside of the injection cylinder device 95 is pressurized by boost cylinder device 96 with a large diameter to a high pressure to boost the molten material 92 filled in the mold cavity 91 through the injection cylinder device 95.
Injection speed level CV and injection pressure level CP during injection process and boosting process in the die-casting machine are shown in FIG. 7. In the figure, the injection cylinder device 95 advances initially at a low speed VL and fills the molten material rapidly at a high speed VH from time point t1 which is braked by virtue of filling pressure of the molten material 92 in accordance with completion of filling process. The boost cylinder 96 is actuated at time point t2 to pressurize the molten material 92 so that the molten material 92 in the mold cavity 91 reaches pressure PH. The injection cylinder 95 is further advanced and is stopped at time point t3 when solidification of the molten material is completed.
For linking control of the injection cylinder device 95 and the boost cylinder device 96 (switching process from the injection process and the boosting process), sequence-valve method for detecting injection pressure fluctuation to conduct switch and limit-switch method for detecting the advancing position of the injection plunger have been used.
Following hydraulic circuit is used in the above sequence valve method.
In FIG. 8, an injection hydraulic circuit 114 from a check valve 111 and injection speed control valve 112 to an accumulator 113 is connected to the injection cylinder device 95. On the other hand, a boost hydraulic circuit 117 reaching the accumulator 113 through a pilot operation boost control valve 116 to be opened and shut by a sequence valve 115.
The sequence valve 115 opens the boost control valve 116 when the pressure of the injection hydraulic circuit 114 exceeds a predetermined boost initiation pressure. Accordingly, when advancement of the injection cylinder device 95 is started by operating the injection speed control valve 112 for conducting the injection process and filling pressure is increased in accordance with completion of filling molten material into the mold die to reach the predetermined boost initiation pressure, the sequence valve 115 is actuated to open the boost control valve 116 to start advancement of the boost cylinder device 96, thereby conducting boosting process.
As shown in FIG. 9, the injection cylinder device 95 has an injection piston 95A, which is advanced by hydraulic pressure of hydraulic oil supplied to a backside thereof by the injection hydraulic circuit 114. Flow-rate of the hydraulic oil from the injection hydraulic circuit 114 is controlled by the injection speed control valve 112 to switch advancement and suspension of the injection piston 95A and adjust advancing speed thereof.
The boost cylinder device 96 has a boost piston 96A thereinside, which is advanced by hydraulic pressure of the hydraulic oil supplied to a backside thereof from the boost hydraulic circuit 117 to pressurize the injection piston 95A from backside through an intermediate member 95B of the injection cylinder device 95. The flow of the hydraulic oil from the boost hydraulic circuit 117 is controlled by the boost control valve 116, which switches advancement and suspension of the boost piston 96A.
On-off operation of the boost control valve 116 is conducted by the sequence valve 115. An electrovalve or the like for switching in response to filling pressure by an appropriate means is suitably used for the sequence valve 115.
Incidentally, in the above-described double-stage die-casting machine, the flow-rate of the hydraulic oil given to the boost cylinder device stays constant without being variably controlled in the boosting process for boosting the injection cylinder device by the boost cylinder device. This is because on-off operation of the hydraulic oil is conducted by the boost control valve and an on-off two-position switching type constant flow valve is conventionally used for the boost control valve.
Since the hydraulic oil is supplied at a constant flow-rate to the boost cylinder device, boost characteristics of casting pressure becomes the injection pressure level CP shown in FIG. 7, in which increase curve is represented as a quadratic curve being less gradient as approaching to maximum pressure PH. This is because the injection plunger receives much resistance in accordance with solidification of the molten material in the mold die and the boost is blunted. More specifically, the casting pressure P is in proportion to square root of product of elapsed time t and parameter a.
On the other hand, burr critical boost curve is known in boosting process. The burr is generated when an excessive pressure is applied to the mold die in boosting process etc. to leak the molten material from parting surface of the mold die. The burr critical boost curve is given as a curve CX where the casting pressure P is in proportion to a product of square of elapsed time t and parameter a (see FIG. 7).
The burr critical boost curve CX becomes such quadratic curve because the molten material in the mold die is not solidified at the initial stage of boosting process and fluidity of the molten material is high enough to leak the molten material from the parting surface, which prevents high pressure from being applied. On the other hand, when the molten material is solidified in accordance with elapsed time, the leakage to the parting line is hard to be caused, so the burr is not likely to be generated when high pressure is applied.
However, in the conventional die-casting machine where the hydraulic oil is supplied to the boost cylinder device at a constant flow-rate, the boost characteristic of the casting pressure is difficult to be approximated to the burr critical boost curve, so that burrs are frequently generated in conducting high-speed casting or using a mold die having low parting surface accuracy, which makes it impossible to manufacture die-casting products with high-quality.